I don't know of anyone else reporting that problem, greenfield. The parts list I checked (Evolution 2000 Mk II, 16-32) shows the entire rear axle being serviced as an assembly, not part by part. The sprocket, including pivots for the planetary gears, looks as if it may be welded to the longer axle shaft, and the other side plate of the differential may carry a bearing for the shorter axle shaft.
It is important that the sprocket runs true, even though it rotates rather slowly in this case. I would suggest a less complicated and simpler machining job than you had in mind. It involves making an additional side-plate for the differential on the sprocket side, so it picks up the existing mounting bolts, which would need to be slightly longer so that the bolts can pass through the existing sprocket as well as the new additional side plate. The outside diameter of the of the additional side plate would be 1" smaller than the OD of the sprocket. Using a lathe, turn off the old sprocket at the outside diameter of the differential. Bore a replacement sprocket to fit closely over the outside of the differential. Bolt the new sprocket to the additional side plate. From then onward, when you wanted to replace the large sprocket it would not involve any special machining except boring the new one to differential-size, and drilling holes for the attaching bolts. You can make the new additional side plate of any thickness you choose so long as it is substantially greater than the thickness of the existing sprocket. The side plate is simply a flat disk, bored on the inside to clear the axle shaft weld, and turned on the outside to 1" smaller diameter than the sprocket. It will have to be drilled accurately for the differential bolts, since they will be what makes it run true: to get a quality job, those holes should be drilled or bored on a rotary table on a milling machine, and they must be a close fit to the differential bolts, not a normal clearance fit. Alternatively of course you could bore the new side plate to be a close fit on the axle, then bore it from one side only to clear the weld attaching the old sprocket to the axle. If you did that the bolt holes for the differential bolts would not need to be especially accurate.
